2.7

Question 1

DNA replication is a…

Answer 1

semi-conservative process

Question 2

when a new double-stranded DNA molecule is formed:

Answer 2

• One strand will be from the original template molecule

- One strand will be newly synthesised

Question 3

DNA replication occurs because

Answer 3

each nitrogenous base can only pair with its complementary partner (CG) (AT)

Question 4

Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase:

Answer 4

• Each new strand formed will be identical to the original strand separated from the template
• The two semi-conservative molecules formed will have an identical base sequence to the original molecule

Question 5

The theory that DNA replication was semi-conservative was confirmed by which experiment?

Answer 5

Meselson-Stahl experiment in 1958

Question 6

Prior to this experiment, three hypotheses had been proposed for the method of replication of DNA:

Answer 6

• Conservative Model
• Semi-Conservative Model
• Dispersive Model

Question 7

Conservative Model

Answer 7

An entirely new molecule is synthesised from a DNA template (which remains unaltered)

Question 8

Semi-Conservative Model

Answer 8

Each new molecule consists of one newly synthesised strand and one template strand

Question 9

Dispersive Model

Answer 9

New molecules are made of segments of new and old DNA

Question 10

Meselson and Stahl were able to experimentally test the validity of these three models using…

Answer 10

radioactive isotopes of nitrogen

Question 11

Meselson and Stahl procedure with nitrogen

Answer 11

DNA molecules were prepared using the heavier 15N and then induced to replicate in the presence of the lighter 14N and DNA samples were then separated via centrifugation to determine the composition of DNA in the replicated molecules

Question 12

The results after two divisions supported the semi-conservative model of DNA replication

Answer 12

• After one division, DNA molecules were found to contain a mix of 15N and 14N, disproving the conservative model
• After two divisions, some molecules of DNA were found to consist solely of 14N, disproving the dispersive model

Question 13

The process of DNA replication is coordinated by two key enzymes

Answer 13

helicase and DNA polymerase

Question 14

Helicase

Answer 14

• Helicase unwinds the double helix and separates the two polynucleotide strands
• It does this by breaking the hydrogen bonds that exist between complementary base pairs
• The two separated polynucleotide strands will act as templates for the synthesis of new complementary strands

Question 15

DNA Polymerase

Answer 15

• DNA polymerase synthesises new strands from the two parental template strands
• Free deoxynucleoside triphosphates (nucleotides with 3 phosphate groups) align opposite their complementary base partner
• DNA polymerase cleaves the two excess phosphates and uses the energy released to link the nucleotide to the new strand

Question 16

The polymerase chain reaction (PCR)

Answer 16

is an artificial method of replicating DNA under laboratory conditions

Question 17

PCR is used to

Answer 17

amplify large quantities of a specific sequence of DNA from an initial minute sample. Each reaction doubles the amount of DNA – a standard PCR sequence of 30 cycles creates over 1 billion copies (230)

Question 18

PCR occurs in a thermal cycler and uses variations in temperature to control the replication process via three steps:

Answer 18

1. Denaturation – DNA sample is heated (~90ºC) to separate the two strands
2. Annealing – Sample is cooled (~55ºC) to allow primers to anneal (primers designate sequence to be copied)
3. Elongation – Sample is heated to the optimal temperature for a heat-tolerant polymerase (Taq) to function (~75ºC)

Question 19

Taq polymerase is an enzyme isolated from

Answer 19

the thermophilic bacterium Thermus aquaticus

Question 20

Thermus aquaticus optimal temperature

Answer 20

As this enzyme’s optimal temperature is ~75ºC, it is able to function at the high temperatures used in PCR without denaturing

Question 21

Transcription

Answer 21

the process by which an RNA sequence is produced from a DNA template

Question 22

transcription: role of DNA polymerase

Answer 22

RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands

Question 23

transcription: role of ribonucleoside triphosphates

Answer 23

When the DNA strands are separated, ribonucleoside triphosphates align opposite their exposed complementary base partner

Question 24

transcription: role of RNA polyermase

Answer 24

RNA polymerase removes the additional phosphate groups and uses the energy from this cleavage to covalently join the nucleotide to the growing sequence

Question 25

transcription: final part

Answer 25

Once the RNA sequence has been synthesised, RNA polymerase detaches from the DNA molecule and the double helix reforms

Question 26

Gene

Answer 26

The sequence of DNA that is transcribed into RNA is called a gene

Question 27

antisense strand

Answer 27

The strand that is transcribed is called the antisense strand and is complementary to the RNA sequence

Question 28

sense strand

Answer 28

The strand that is not transcribed is called the sense strand and is identical to the RNA sequence (with T instead of U)

Question 29

Transcription of genes occur in the...

Answer 29

nucleus (where DNA is), before the RNA moves to the cytoplasm (for translation)

Question 30

The base sequence of an mRNA molecule encodes the...

Answer 30

production of a polypeptide

Question 31

The mRNA sequence is read by the ribosome in triplets of bases called...

Answer 31

codons

Question 32

Each codon codes for...

Answer 32

one amino acid with a polypeptide chain

Question 33

The order of the codons in an mRNA sequence determines the order of...

Answer 33

amino acids in a polypeptide chain

Question 34

The genetic code is the set of rules by which...

Answer 34

information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells

Question 35

The genetic code identifies the...

Answer 35

corresponding amino acid for each codon combination

Question 36

how many codon possibilities are there?

Answer 36

As there are four possible bases in a nucleotide sequence, and three bases per codon, there are 64 codon possibilities (43)

Question 37

The coding region of an mRNA sequence always begins with a...

Answer 37

START codon (AUG) and terminates with a STOP codon

Question 38

Typically the genetic code shows the codon combinations expressed on an...

Answer 38

mRNA molecule

Question 39

translation is the process of...

Answer 39

protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain

Question 40

translation: role of ribosomes (binding)

Answer 40

Ribosomes bind to mRNA in the cytoplasm and move along the molecule in a 5’ – 3’ direction until it reaches a start codon (AUG)

Question 41

translation: role of anticodons

Answer 41

Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. AUG = UAC)

Question 42

translation: role of each tRNA molecule

Answer 42

Each tRNA molecule carries a specific amino acid (according to the genetic code)

Question 43

translation: role of ribosomes (catalyse)

Answer 43

Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via condensation reactions)

Question 44

translation: when does stop codon occur

Answer 44

The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon

Question 45

what happens when translation ceases?

Answer 45

At this point translation ceases and the polypeptide chain is released

Question 46

Translation Mnemonic

Answer 46

Mr Cat App

Question 47

Translation Mnemonic meaning

Answer 47

``` Messenger RNA (goes to…) Ribosome (reads sequence in …) Codons (recognised by …) Anticodons (found on …) Transfer RNA (which carries …) Amino acids (which join via …) Peptide bonds (to form …) Polypeptides ```

Question 48

The genetic code is?

Answer 48

universal – almost every living organism uses the same code (there are a few rare and minor exceptions)

Question 49

The ability to transfer genes between species has been utilised to produce...

Answer 49

human insulin in bacteria

Question 50

insulin mass production process

Answer 50

The gene responsible for insulin production is extracted from a human cell It is spliced into a plasmid vector (for autonomous replication and expression) before being inserted into a bacterial cell The transgenic bacteria (typically E. coli) are then selected and cultured in a fermentation tank (to increase bacterial numbers) The bacteria now produce human insulin, which is harvested, purified and packaged for human use (i.e. by diabetics)

Question 51

insulin mass production name

Answer 51

Recombinant Gene Transfer

Question 52

mRNA is a complementary copy of a...

Answer 52

DNA segment (gene) and consequently can be used to deduce the gene sequence

Question 53

mRNA → DNA | For converting a sequence from mRNA to the original DNA code, apply the rules of complementary base pairing:

Answer 53

Cytosine (C) is replaced with Guanine (G) – and vice versa Uracil (U) is replaced by Adenine (A) Adenine (A) is replaced by Thymine (T)

Question 54

mRNA → Polypeptide | In order to translate an mRNA sequence into a polypeptide chain, it is important to establish the correct reading frame

Answer 54

The mRNA transcript is organised into triplets of bases called codons, and as such three different reading frames exists - An open reading frame starts with AUG and will continue in triplets to a termination codon - A blocked reading frame may be frequently interrupted by termination codons Once the start codon (AUG) has been located and reading frame established, the corresponding amino acid sequence can be deduced using the genetic code

Question 55

open reading frame

Answer 55

An open reading frame starts with AUG and will continue in triplets to a termination codon

Question 56

blocked reading frame

Answer 56

A blocked reading frame may be frequently interrupted by termination codons